The simple answer is that these are not duplicate emails, one message is sent to the user directly and the other is sent to a distribution list and that is forwarded to the users mailbox - they may have the same content but they are initially addressed to different users. Have a look at the headers of both emails, one will have the To: address of the distribution list and the other will have the To: address of the user, both emails will have different Message-ID: and therefore not duplicates.